Animal-Like Protista

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Animal-Like Protista
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• Introduction
• Protists are eukaryotes and include unicellular
  (e.g., amoeba) and multicellular forms (e.g.,
  algae)

Filamentous algae
Eukaryotic cell

• The word protista (Gr. protos, very first,
  ktistos, to establish) implies great antiquity
• Protista represent a polyphyletic group
• Two interesting scenarios regarding the history
  of life on earth emerged during the evolution of
  protists the origin of the eukaryotic cell and
  the subsequent emergence of multicellular
  eukaryotes

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• The Emergence of the Eukaryotic Cell
• The small size and simpler construction of the
  prokaryotic cell has many advantages but also
  imposes a number of limitations
• Number of metabolic activities that can occur at
  any one time is smaller
• Smaller size of the prokaryotic genome limits
  the number of genes which code for enzymes
  controlling activities
• Natural selection resulted in increasing
  complexity in some groups of prokaryotes two
  major trends were apparent

1. Toward multicellular forms such as
cyanobacteria different cell types with
specialized functions
Anabaena
2. The compartmentalization of different
functions within cells the first eukaryotes
resulted from this solution
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• The Evolution of Eukaryotes
• The evolution of the compartmentalized nature of
  eukaryotic cells may have resulted from two
  processes
• 1. Specialization of plasma membrane
  invaginations.

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The Evolution of Eukaryotes cont.
2. Endosymbiotic associations of prokaryotes may
have resulted in the appearance of some
organelles. Mitochondria, chloroplasts, and some
other organelles evolved from prokaryotes living
within other prokaryotic cells
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• The Endosymbiotic Theory
• The endosymbiotic theory was developed by Lynn
  Margulis
• It proposes that certain prokaryotic species,
  called endosymbionts lived within larger
  prokaryotes
• Chloroplasts are believed to have descended from
  endosymbiotic photosynthesizng prokaryotes, such
  as cyanobacteria, living in larger cells
• Mitochondria are postulated to be descendents of
  prokaryotic areobic heterotrophs.
• Perhaps they gained entry as parasites or
  undigested prey of larger prokaryotes.
• The association may then have progressed from
  parasitism or predation to mutualism.
• And, as the host and endosymbiont became more
  interdependent, they integrated into a single
  organism

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• Evidence for the Endosymbiotic Theory
• Chloroplasts and mitochondria are of the
  appropriate size to be descendents of eubacteria
• Their inner membranes contain several enzymes
  and transport systems similar to those of
  prokaryotic plasma membranes
• They replicate by splitting processes similar to
  binary fission present in prokaryotes
• They have DNA which is circular and not
  associated with histones or other proteins, as in
  prokaryotes
• They contain their own tRNA, ribosomes and other
  components for DNA transcription and translation
  into proteins
• Chloroplasts have ribosomes more similar to
  prokaryotic ribosomes (with regards to size,
  biochemical characters, etc) than to eukaryotic
  ribosomes mitochondrial ribosomes vary, but are
  also more similar to prokaryotic ribosomes
• rRNA of chloroplasts is more similar in basic
  sequence to that of certain photosynthetic
  eubacteria than to rRNA in eukaryotic cytoplasm
  also, chlorplast rRNA is transcribed from genes
  in the chloropast while eukaryotic rRNA is
  transcribed from nuclear DNA

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• Archezoans and Early Evolution of Eukaryotes
• An ancient lineage of eukaryotes branched away
  from the eukaryotic tree very early, perhaps as
  long as 2 billion years ago
• The group is referred to as the archezoa and
  contains only a few phyla
• Lack mitochondria and plastids, and have
  relatively simple cytoskeletons
• Their ribosomes have some characteristics more
  closely aligned with prokaryotes than with
  eukaryote
• Giardia intestinalis is a modern representative
  of an archezoan
• If organisms like Giardia diverged from the
  eukaryotic lineage before the process of nuclear
  fusion and meiosis evolved, their dual nuclei may
  be a clue to the past
• This coupled with the absence of mitochondria in
  this group and other archezoan is consistent with
  an origin occurring before the endosymbiotic
  relations that gave rise to the mitochondria in
  aerobic species

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Overview of the Tree of Life
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• Introduction to Protozoan Protists
• Protozoans (Gr. proto first zoa animal) are
  the single-celled animal-like members of the
  kingdom Protista
• They are clearly eukaryotes, e,g., distinct
  nuclei, membrane bound organelles, etc. unlike
  animals, never develop from a blastula
• Remarkably diverse in terms of size, morphology,
  mode of nutrition, locomotory mechanism, and
  reproductive biology
• Protozoans are regarded as being a polyphyletic
  group

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General Characteristics of Protozoan Protists

• Entire organism is bounded by the plasmalemma
  (cell membrane)
• The cytoplasm is often differentiated into a
  clear, outer gelatinous region, the ectoplasm,
  and an inner, more fluid region fluid or sol
  state, the endoplasm
• Many organelles are typical of most
  multicellular metazoan cells
• However, many protozoans contain organelles not
  generally found among the metazoa, e.g.,
  contractile vacuoles and trichocysts

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• Cilia and Flagella
• Locomotor appendages that protrude from the
  protozoan cell
• Cilia are shorter and more numerous, whereas,
  flagella are longer and less less numerous
• Cilia and flagella are similar structurally
  microtubules are arranged in a ring of 9
  microtubule doublets surrounding a central pair
  of microtubles (92 arrangement) microtubules
  are covered by an extension of the plasma
  membrane they are anchored to the cell by a
  basal body
• Cilia and flagella differ in their beating
  patterns

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• Pseudopodia
• When organisms like amoeba are feeding and
  moving, they form temporary cell extensions
  called pseudopodia
• The most familiar form are called lobopodia
  contain ectoplasm and endoplasm used for
  locomotion and engulfing food
• When a lobopodium forms, an extension of the
  ectoplasm called the hyaline cap appears and
  endoplasm flows into this cap
• As the endoplasm moves into the cap it fountains
  out and it changes from the fluid state to the
  gel state (endoplasm to ectoplasm)
• Pseudopodium anchors to the substrate and the
  cell is drawn forward

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• Nutrition and Digestion
• Ingested food particles generally become
  surrounded by a membrane, forming a distinct food
  vacuole digestion is entirely intracellular
• Vacuoles move about in the fluid cytoplasm and
  the contents are broken down by enzymes
• The contents of the vacuoles can change, e.g.,
  go from acidic to basic
• This is important because digestion for these
  organisms requires exposing the food to a series
  of enzymes, each of which has a specific role
  that operates under a narrow range of pH
• Controlled changes of pH that occur within the
  food vacuoles allow for the sequential
  disassembly of foods
• Once solubilized, nutrients move across the
  vacuole wall and into the endoplasm of the cell
• Indigestible solid wastes are commonly
  discharged to the outside through an opening in
  the plasma membrane

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• Excretion and Osmoregulation
• Contractile vacuoles are organelles involved in
  expelling water from the cytoplasm
• Fluid is collected from the cytoplasm by a
  system of membranous vesicles and tubules called
  spongiome tubules
• The collected fluid is transferred to a
  contractile vacuole and is subsequently
  discharged to the outside through a pore in the
  cell membrane
• Vacuoles are most commonly found among
  freshwater species

pore
Spongiome tubules
ampulla
Vacuole
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• Reproduction
• Asexual reproduction is commonly encountered
  among protozoans
• Some reproduce asexually through fission, a
  controlled mitotic replication of chromosomes and
  splitting of the parent into two or more parts
• Binary fission - protozoan splits into two
  individuals
• Multiple fission many nuclear divisions precede
  the rapid differentiation of the cytoplasm into
  many distinct individuals
• Budding a portion of the parent breaks off and
  differentiates into a new individual

fission
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• Reproduction cont.
• Many protozoans possess the capacity for
  regeneration
• For example, encystment and excystment exhibited
  by freshwater and parasitic species
• During encystment, substantial dedifferentiation
  of the organism occurs, forming a cyst compact,
  expels excess water, forms a gelatinous covering
  is secreted
• The cyst can withstand long periods of exposure
  to what would otherwise be intolerable conditions
  of acidity, thermal stress, dryness, etc.
• Once conditions improve excystment ensues with
  the regeneration of all former internal and
  external structures

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• Classification
• Phylum Sarcomastigophora
• Move by means of flagella and/or pseudopodia
  possess a single type of nucleus.
• Subphylum Mastigophora
• Locomotion is by means of one of more
  flagella.
• Class Phytomastigophorea or
  phytoflagellates autotrophic
• forms containing chlorophyll one or two
  flagella,
• e.g., dinoflagellates Euglena Volvox
• Class Zoomastigophorea or zooflagellates
  heterotrophic forms,
• e.g., trypanosmes that parasitize humans and
  cause sleeping
• sickness tsetse flies serve as vectors

Blood cell
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• Subphylum Sarcodina
• Mostly marine, but some inhabit freshwater and
  soil some are parasitic
• Use pseudopodia for feeding and locomotion.
• Feed by a process known as phagocytosis.
• A number of species of sarcodines possess a
  protective outer shell or test, e.g. the
  radiolarians (silica) and foraminiferans (calcium
  carbonate)
• Both the radiolarians and the forams feed by
  extending their pseuopodia through openings in
  the shell.

Foraminiferan
Amoeba
Radiolarian
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• Phylum Ciliophora
• Exclusive to freshwater
• Cilia or ciliary organelles present in at least
  one stage of the life cycle.
• The ciliates are unique in that they possess 2
  kinds of nuclei a large macronucleus and one or
  more smaller micronuclei.
• The macronucleus controls the normal metabolism
  of the cell, while the micronuclei are concerned
  with sexual reproduction.

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• Ciliophoran Reproduction
• Asexually via binary fission sexually via
  conjugation.
• 2 individual align and partially fuse all but
  one micronucleus in each cell disintegrates.
• The partners swap one micronucleus this
  micronucleus then fuses to another micronucleus,
  forming a diploid organism with genetic material
  from the 2 individuals.

conjugation
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• Phylum Apicomplexa
• This is an exclusively parasitic group of
  protozoans that lack locomotory organelles,
  except during certain reproductive stages.
• They possess a characteristic set or organelles
  called the apical complex, which aids in
  penetrating host cells.
• Includes parasites that cause malaria (e.g.,
  Plasmodium) to humans mosquitoes serve as vectors

apical complex